Abstract

The role of physiology in mediating the growth/predation risk trade-off has been largely ignored. We examined effects of predation risk on relationships between growth and storage molecules in Enallagma aspersum and Ischnura verticalis damselfly larvae that differ in this trade-off. In laboratory and field experiments, both species had similar growth and mortality rates and similar concentrations of storage molecules in the absence of mortality threats. However, in the presence of dragonfly predators Ischnura larvae had higher mortality rates and grew faster than Enallagma larvae. Consistent with the difference in growth rate, Enallagma's total protein concentrations decreased under predation risk while those of Ischnura did not. Glucose and glycogen concentrations were not affected, while triglyceride concentrations were lower under predation risk in Enallagma but not in Ischnura. Species differences at the physiological level to the presence of mortality threats may be crucial to understanding patterns in metamorphic and post-metamorphic traits.